1. Technical Field
The present invention relates to a medium access method for contention and non-contention and, more particularly, to a medium access method for contention and non-contention, in which a poll number for commercial stations (wireless stations for communication) is managed by a base station according to the IEEE 802.11e standard, thereby guaranteeing smooth traffic and improved Quality of Service (QoS).
2. Related Art
In general, a wireless local area network (LAN) is a mechanism for performing communication on a network by use of radio waves rather than cables. The wireless LAN was formed as an alternative for solving various difficulties in installation, maintenance or repair, mobility and so forth which are caused by cabling, and a solution to those difficulties has become more of a necessity due to an increase in the number of users.
The wireless LAN consists of an access point (hereinafter, referred to as “AP”) and a wireless station. The AP is equipment for sending radio waves to enable users of the wireless LAN, within a transmission distance to get access to the Internet, to use the network. The AP serves as a base station for a mobile phone or as the hub for a wired network. For example equipment called “AP” (or network interface toll center) has been already installed within a service area for wireless high-speed Internet service provided by an Internet Service Provider (ISP).
The wireless station should be provided with a wireless LAN card or the like in order to perform wireless network communication, and includes a PC or Personal Computer (inclusive of a notebook computer), a Personal Digital Assistant or PDA, and so forth.
However, currently, a study is being actively conducted in order to accommodate various services demanding real-time characteristics, such as voice communication services, multilateral video conference services, real-time image transmission services, and so forth. Thus, wireless LAN telephones are currently commercialized, and they enable anyone having access to the wireless LAN to make and receive calls.
The wireless LAN should be capable of guaranteeing QoS to stations or users using such services so as to smoothly provide various application services requiring the real-time characteristics. Further, since each of the stations connected to the wireless LAN makes a request for a different level of service, the wireless LAN should be capable of providing optimal services to the respective stations as well.
Standards for the wireless LAN which are presently used widely have an imminent function capable of guaranteeing QoS or Class of Service (CoS), or have a procedure for compensating related functions. The wireless LAN standard of the Institute of Electrical and Electronics Engineers (IEEE), which is widely applied to various countries including North America and Korea, supports a Point Coordination Function (PCF) as an option in order to make it possible to transmit real-time information, wherein the PCF refers to a medium access control function according to a polling mechanism.
The wireless LAN standard of IEEE follows “Standard for Information Technology-Telecommunications and Information Exchange between Systems-Local and Metropolitan Area Networks-Specific Requirements-part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications,” 1999 Edition.
Hereinafter, the wireless LAN standard of IEEE is referred to, in short, as the IEEE 802.11 standard. This standard defines the media access control (MAC) and physical (PHY) layers for the wireless LAN.
The MAC layer defines orders and rules which a station or apparatus using the shared medium must observe in the use or access of the shared medium, thereby adjusting use of a capacity of the network and determining which of the stations is allowed to use the medium for transmission.
A MAC protocol includes an enhanced distributed coordination function (EDCF) for providing distributed coordination function (DCF) and QoS, and a hybrid coordination function (HCF) for providing point coordination function (PCF) and QoS. The DCF serves to transmit a MAC Protocol Data Unit (MPDU) through contention, wherein a period of transmitting the MPDU is called a contention period (CP). The PCF serves to transmit the MPDU through contention and polling, especially through polling, wherein the period of transmitting the MPDU is called a non-contention period, hereinafter referred to as a “contention-free period” or “CFP”.
This DCF mechanism is an access control mechanism defined as a fundamental specification in the IEEE 802.11 standard, which provides authority to allow one of a plurality of stations to access a medium through contention according to a contention-based algorithm known as Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). In the station that does not gain access to the medium, a Network Allocation Vector (NAV) is set to be in a standby state.
Here, the NAV is provided through detection of a virtual carrier. Most of IEEE 802.11 frames include a duration field, which is used in reserving the medium for a fixed time. The NAV is a timer representing time information when the medium is reserved. Any one of the stations forces an expected time for use of the medium, including all the frames needed to complete current operation, to be set for the NAV, and other stations wait for the NAV to stand at 0 (null). If the NAV does not stand at 0 (null), the virtual carrier detection indicates that the medium is in use or busy. By contrast, if the NAV stands at 0 (null), the virtual carrier detection indicates that the medium is available. This NAV can be set by Request to Send (RTS), Clear to Send (CTS), and Acknowledgment (ACK) which are used to exchange the frames. In general, information capable of setting the NAV is transmitted, in addition to a header of the RTS or CTS frame.
As to the DCF mechanism, when any one of the stations is accessed within the CP through contention between the stations, other stations are delayed in getting access to the medium by the NAV time. After the NAV time, any one of the other stations gains an access chance through contention between the other stations.
In other words, the station checks whether the medium is busy or not. If so, the station waits for a predetermined time. After the predetermined time, if the medium is not busy, i.e. idle, the station decreases a backoff time. In this manner, the predetermined time for which each of the stations waits in order to initiate a traffic is called an InterFrame Space (IFS). There are three IFSs for the MAC protocol traffic: DIFS referring to a DCF interframe space, PIFS referring to a PCF interframe space, and SIFS referring to a short interframe space.
Before transmitting the frame, the station employing the DCF mechanism determines whether the medium is busy. If the medium is idle for a time longer than or equal to the DIFS, the station can transmit the frame. By contrast, if the medium is busy, the station initiates the backoff procedure. Then, when the backoff timer has a value of 0 (null), the station occupies the medium to transmit the frame.
However, because the DCF mechanism is based on acquisition of a transmission chance caused by a probability of getting access to the medium through contention between the stations, it is not easy to support services restricted by time. Further, when the number of stations is gradually increased, this interferes with services such as multimedia services where throughput should be constantly maintained. In addition, overhead is generated within a preset beacon interval to make it impossible to guarantee the QoS.
In the wireless LAN, the QoS is guaranteed by a contention-free medium access method, which is a representative polling-based mechanism, and the PCF uses this method. According to the single polling mechanism by the PCF or HCF, a plurality of stations can be connected to the AP without contention through the medium. In other words, when a single poll is generated from the AP toward the plurality of stations, a preset one of the stations is connected. Then, when the next single poll is generated, one of the other stations which are on standby with the first priority is connected.
However, the poll (or polling message) should be sent whenever the stations are polled, and there are portions of performing separate timing management for the CFP. For these reasons, there is a problem in that it is complicated in realization.
Meanwhile, technology related to a multi-polling DCF mechanism for overcoming disadvantages of the single polling mechanism by the PCF using basic functions of the DCF is disclosed in Korean Patent Registration Publication No. 10-0442821 (filed on Jul. 23, 2004 and titled “Data Communication Method based on Backoff Number Control”).
As to the multi-polling DCF mechanism disclosed in the prior patent, when a multi-polling message, which includes information on identifiers (IDs) of stations intended for polling and on arbitrary backoff numbers allocated to the respective stations, is transmitted from an AP, the corresponding station receives the multi-polling message so as to set the backoff number allocated thereto in a backoff timer thereof, and subsequently performs a backoff procedure to attempt to get access to a medium. At this time, the station is on a polling list according to the multi-polling message, and has authority to get access to the medium on the basis of information on the backoff number.
In this manner, the multi-polling DCF mechanism transmits one polling message defining the backoff numbers of the corresponding stations to a plurality of stations requiring the QoS (hereinafter, referred to as “MP-DCF stations”) by use of a multi-poll or a beacon, thereby making it possible to equally endow each of the MP-DCF stations with a transmission chance.
However, in the MP-DCF used as a polling-based medium access control mechanism for guaranteeing the QoS as set forth above, there are problems to be solved with respect to access to the medium.
First, in the medium access mechanism, such as the MP-DCF mechanism, only the MP-DCF stations are equally provided with a transmission chance on the basis of non-contention, but commercial stations employing the medium access mechanism, such as the DCF mechanism, are not provided with a transmission chance on the basis of contention. For this reason, the MP-DCF medium access mechanism does not allow commercial stations to coexist on the basis of the non-contention.
Further, the commercial stations cannot recognize the multi-polling message and provide a chance to get access to the medium at an interval of the DIFS between the data, so that the medium access of the MP-DCF stations cannot be guaranteed for the CFP due to interference with the commercial stations.
Therefore, when the MP-DCF stations coexist with the commercial stations, a collision phenomenon occurs because these two kinds of stations attempt to get first access to the medium. As such, the probability of failure to get access to the medium is increased, so that it is difficult to guarantee QoS.